1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an apparatus and method for efficiently transmitting reverse packet data in a mobile communication system.
2. Description of the Related Art
Conventionally, data transmission in a mobile communication system can be divided into a forward data transmission and a reverse data transmission. The forward data transmission is a data transmission from a base station to a mobile terminal, while the reverse data transmission is a data transmission from the mobile terminal to the base station. A data transmission form can be divided into categories supporting only voice service, supporting voice service and simple data service, supporting only high-speed data service, and simultaneously supporting multimedia service and voice service according to type of data transmitted from the mobile communication system. Mobile communication systems providing the data service described above are designed to process a large amount of information at a fast rate.
The mobile communication system for simultaneously processing the multimedia service and voice service supports the multimedia service using the same frequency band. Moreover, a plurality of users can simultaneously transmit data by means of code division multiple access (CDMA) in the mobile communication system. In this case, the users can be identified by unique numbers allocated thereto in the CDMA. In the CDMA, the reverse data transmission is performed through a packet data channel in units of a physical layer packet (RLP). The length of the packet is fixed according to the data transmission rate. The packet data transmission rate is variable for each packet. The transmission rate of each packet is controlled by the power of the mobile terminal, an amount of data to be transmitted, a power control bit, etc. The power control bit is control information transmitted through a forward rate control channel (F-RCCH) from the base station.
Furthermore, the mobile communication system can carry out retransmission in a physical layer to improve the reverse throughput. According to the retransmission in the physical layer, an acknowledgement/non-acknowledgement (ACK/NACK) signal is sent according to the presence of a packet error (e.g., cyclic redundancy check (CRC)) after the base station demodulates the received reverse data packet, and the mobile terminal receives the ACK/NACK signal to determine whether a previously transmitted packet must be retransmitted or a new packet must be transmitted. The physical layer's retransmission process is called hybrid auto repeat and request (HARQ). According to the retransmission process in the physical layer, the base station demodulates the received reverse data packet, and the ACK/NACK signal of the physical layer is sent according to a packet error or CRC. If the ACK signal has been received from the base station, the mobile terminal determines that the previously transmitted packet has been successfully received and transmits a new packet. Otherwise, if the NACK signal has been received from the base station, the mobile terminal determines that the transmitted packet has not been successfully received and retransmits the previous packet.
On the other hand, a method for controlling reply signal reception in the conventional mobile communication system will be described with reference to the flow chart shown in FIG. 1.
First, the reply signal receiver is in a standby state at step 101. Subsequently, the reply signal receiver determines, at step 102, whether or not the ACK signal has been received from a reply signal transmitter. If the ACK signal has been received, the reply signal receiver determines that a previously transmitted packet has been successfully received from the reply signal transmitter and transmits new packet data at step 103. However, if the ACK signal has not been received, the reply signal receiver determines, at step 104, whether or not the NACK signal has been received from the reply signal transmitter. If the NACK signal has been received, the reply signal receiver determines that the previously transmitted packet has not been successfully received by the reply signal transmitter, and retransmits the previously transmitted packet at step 105. If neither the ACK nor the NACK signal had been received, the process returns to the standby state of step 101.
A method for controlling reply signal transmission in the conventional mobile communication system will be described with reference to the flow chart shown in FIG. 2.
The reply signal transmitter is in a standby state at step 201. The reply signal transmitter determines, at step 202, whether or not the packet data has been received from the reply signal receiver. If the packet data has not been received, the reply signal transmitter proceeds to the above step 201 so that it can maintain the standby state before the packet data is received. However, if the packet data has been received, the reply signal transmitter demodulates the packet data at step 203. Subsequently, the reply signal transmitter determines, at step 204, whether or not an error is present in the received packet data. If an error is present in the packet data, the reply signal transmitter transmits the NACK signal to the reply signal receiver so that a packet data retransmission request is made at step 205. However, if no error is present in the packet data, the reply signal transmitter transmits the ACK signal to the reply signal receiver so that a new data transmission request is made at step 206.
Because an incurable error at the time of transmitting and receiving the ACK/NACK signal as described above is directly associated with throughput of the entire system, high reliability is required. The reliability depends upon the transmission power of an acknowledgement channel that transmits and receives an ACK/NACK signal. Assuming that the reply signal transmitter has successfully received the packet data and then has transmitted the ACK signal to the reply signal receiver, the reply signal receiver may erroneously determine that the NACK signal has been received due to an incurable error at the time of receiving the ACK signal. At this point, the reply signal receiver unnecessarily retransmits the packet already successfully received by the reply signal transmitter. Consequently, there is a waste of radio resources and a degradation of throughput may result.
Assuming that the reply signal transmitter has unsuccessfully received the packet data and then has transmitted the NACK signal for a retransmission request, the reply signal receiver may erroneously determine that the ACK signal has been received due to an incurable error at the time of receiving the NACK signal. At this point, although a transmission error has occurred on a radio link, the reply signal receiver will transmit the next packet to the reply signal transmitter. This causes link-layer retransmission or higher-layer retransmission such as transmission control protocol (TCP) retransmission. Consequently, throughput degradation may occur.
Limited resources used for the forward transmission include electric power of the base station. As the power of the base station increases, the amount of interference affecting an adjacent base station increases. For this reason, the power of the base station must be limited to an appropriate power value. However, a data transmission rate capable of being acquired by the power of the base station is limited. Thus, a forward design of the mobile communication system must be achieved so that the limitation can be overcome.
Interference is a factor limiting the reverse resources. Reverse transmission signals of mobile terminals cause interference with each other. Where a plurality of mobile terminals are coupled to one base station, one mobile terminal performs a transmission operation using a high power level so that a high data transmission rate can be acquired. The base station communicating with the mobile terminal will receive a signal at the high power level from the mobile terminal. The high power causes a large amount of interference to signals of other mobile terminals. If the plurality of mobile terminals simultaneously desire to transmit data at the high transmission rate, a total amount of reverse interference is very high and the probability that the signals of all mobile terminals can be successfully received is lowered. Thus, in order for the throughput of the mobile communication system to be maximized, the base station adjusts the total amount of reverse interference. The base station must efficiently manage data transmission rates of the mobile terminals.
FIG. 3 is a graph explaining the reason why the base station must efficiently adjust reverse transmission of the mobile terminals. A dotted line in FIG. 3 denotes target system load or target rise over thermal (RoT) of the base station. RoT is a ratio between the total power received from all the mobile terminals at the base station and the thermal noise power as one measure of the total reverse interference.
The reason why the base station must manage the target system load or target RoT is as follows. Conventionally, the mobile communication system supports power control so that the quality of a reverse or forward link is ensured. When the base station does not manage reverse system load or the total of received reverse power is below a specific value, the value of the reverse system load or the total of received reverse power may be great. In this case, the amount of interference may also be great. Therefore, signal to noise power ratio (SNRs) associated with the signals of all the mobile terminals are reduced. In this case, in order for communication quality of each link to be maintained, the mobile terminals continuously increase power output. As a vicious cycle causing an increase in the amount of interference is repeated, communication can be disabled. For example, when one mobile terminal associated with a data transmission rate uses high power to acquire a high data transmission rate, an amount of reverse interference due to the mobile terminal greatly increases and other mobile terminals increase power to maintain link quality. As the amount of interference increases, the vicious cycle is repeated. To avoid the above-described vicious cycle, the base station needs to efficiently control data transmission rates of all the mobile terminals so that the reverse load cannot exceed the target system load. Moreover, the base station needs to control all the mobile terminals so that the total of the received reverse signal power does not exceed the target RoT.
As shown in FIG. 3, the reverse interference can be divided into an inter-cell interference, a voice or circuit channel interference, a packet data channel interference, etc.
The inter-cell interference is an interference occurring due to signals received from the mobile terminals communicating with another base station. The amount of inter-cell interference varies with time. Moreover, the inter-cell interference is an interference from the mobile terminals communicating with another base station. The amount of inter-cell interference cannot be controlled or correctly predicted by a reference base station.
The voice or circuit channel interference is an interference caused by another voice channel or circuit-based channel. Because the voice or circuit channel is a channel allocated by the base station, the base station can predict a basic amount of interference. Typically, the voice or circuit channel has a higher priority than the packet data channel.
The packet data channel interference is an interference occurring from a packet data channel. The base station appropriately adjusts the amount of interference occurring from the packet data channel. After the base station subtracts an amount of inter-cell interference and an amount of voice or circuit channel interference from the overall system load or RoT, it allocates the remaining amount of power to the packet data channel. At this point, the base station appropriately adjusts the amount of interference occurring from the packet data channel and performs a control operation so that the entire system load or RoT does not exceed the entire target system load or target RoT.
The base station controls reverse packet data transmission of the mobile terminals, that is, reverse data transmission rates of the mobile terminals, on the basis of the target system load or target RoT, thereby adjusting the total amount of reverse interference.
In the conventional mobile communication system, the base station transmits a rate control bit (RCB) through a forward rate control channel (F-RCCH) so that the data transmission rates of the mobile terminals can be controlled. The RCB has “0”, “+1” or “−1”. When the RCB value is “+1”, the mobile terminal increments its own data transmission rate by one step. When the RCB value is “−1”, the mobile terminal decrements its own data transmission rate by one step in the next transmission interval. When the RCB value is “0”, the mobile terminal maintains its own data transmission rate in the next transmission interval.
In order for the mobile communication system to control the reverse data transmission rates of the mobile terminals, reliability of the RCB transmitted by the base station is equally set in all the mobile terminals irrespective of current data transmission rates of the mobile terminals. Accordingly, there are the following problems.
First, as an example, it is assumed that the RoT of 6 dB operates as the target RoT when the base station controls reverse transmission. This means that the base station performs a control operation so that a value of the entire reverse reception RoT does not exceed 6 dB when deciding a data transmission rate of the reverse packet data channel of each mobile terminal. Reverse RoT management is one of the important elements necessary for maintaining the entire reverse performance. In the case where the base station requests that the mobile terminals simultaneously increment data transmission rates, a scheduling operation of the base station determines whether or not the reverse RoT exceeds 6 dB, when incrementing the data transmission rates of the mobile terminals by one step according to their priorities. If it is determined that the reverse RoT does not exceed 6 dB even though the specific mobile terminal increments the data transmission rate by one step, the base station transmits the RCB so that the data transmission rate of the specific mobile terminal can be incremented by one step. Otherwise, if it is determined that the reverse RoT does exceed 6 dB when the specific mobile terminal increments the data transmission rate by one step, the base station transmits the RCB so that the specific mobile terminal can decrement its own data transmission rate by one step or continuously maintain the current data transmission rate. For example, it is assumed that the RoT measured at a specific time point is 2 dB. When a current data transmission rate of 9.6 kbps in the mobile terminal is incremented by one step, the incremented data transmission rate is 19.2 kbps. In this case, it is assumed that the data transmission rate is incremented by 9.6 kbps and a predicted RoT is 2.0+delta1 dB. On the other hand, when a current data transmission rate of 307.2 kbps in the mobile terminal is incremented by one step, the incremented data transmission rate is 614.4 kbps. In this case, it is assumed that the data transmission rate is incremented by 307.2 kbps and predicted RoT is 2.0+delta2 dB. Delta1 is approximately 32 times as large as delta2 as shown in the following Equation 1.delta2=delta1*307.2 kbps/9.6 kbps  (1)
It can be seen from the above Equation 1 that the target system RoT associated with the RCB is proportional to an increment value of the data transmission rate.
A case where an error is incurred in the RCB will be described. When the mobile terminal increments its own data transmission rate by one step by making an erroneous determination although the base station commands the mobile terminal to decrement the data transmission rate, unpredicted reverse interference occurs and can have a negative effect on the quality of a signal of a different mobile terminal. When the mobile terminal decrements its own data transmission rate by one step by making an erroneous determination although the base station commands the mobile terminal to increment the data transmission rate, reverse resources cannot be completely used.
In order that a determination can be made as to whether or not the effects of an erroneous RCB do not impact the data transmission rates of all the mobile terminals, the next example will be described. It is assumed that the number of mobile terminals trying to perform the reverse transmission to one base station is two at a specific time point. Here, one mobile terminal currently transmits data at a data transmission rate of 9.6 kbps, and the other mobile terminal currently transmits data at a data transmission rate of 307.2 kbps. When an error occurs in the RCB for incrementing the data transmission rate of 9.6 kbps by one step, an RoT error of 2*delta1 will occur. When an error occurs in the RCB for incrementing the data transmission rate of 307.2 kbps by one-step, an RoT error of 32*2*delta1 will occur. Therefore, it can be seen that the amount of reverse interference caused by an error in the RCB is intimately associated with the data transmission rate of the mobile terminal.
As described above, an operation for managing the total amount of reverse interference in the base station plays a very important role in maintaining the reverse performance. The base station adjusts the data transmission rate of each mobile terminal through the RCB so that the total amount of interference can be efficiently managed. If the amount of reverse interference caused by an error in the RCB of the mobile terminal is ignored and the same RCB is set for all the mobile terminals, there is a problem in that power of the base station necessary for transmitting the RCB is inefficiently used and the reverse RoT cannot be managed at a desired level.